Organic radicals play crucial roles in many enzymes, where they are typically formed as electron transfer intermediates in redox reactions. In many cases the radical formation and elimination steps are coupled to the movement of protons, in what is generally termed proton-coupled electron transfer (PCET). The suite of modern EPR techniques provide an ideal basis for probing radical-centered biochemical reactions. In particular, if one is targeting PCET reactions, double resonance techniques such as ENDOR and ESEEM can provide crucial information about the location of magnetic hydrogen nuclei during the radical reaction. For example, the protonation state of the radical and the identity of key hydrogen bond donors or acceptors can be obtained with these methods. We propose multifrequency EPR/ENDOR experiments to target several interesting enzymatic radical intermediates where the electron transfers are coupled (or possibly coupled) to proton transfers. Phycocyanobilin:ferredoxin oxidoreductase (PcyA) is a ferredoxin-dependent bilin reductase involved in the biosynthesis of linear tetrapyrrole prosthetic groups. PcyA carries out two regiospecific vinyl reductions, and we have identified two substrate radical intermediates formed during the course of the reaction. Nitric oxide synthases.(NOS) catalyzes the conversion of L-arginine to citrulline and NO, with N-hydroxy-L-arginine (NHA) as an intermediate. A biopterin radical is transiently formed in the conversion of arginine to NHA, and a similar radical intermediate likely forms in the oxidation of NHA to the final products. Photosystem II contains two redox active tyrosines, YD and YZ, that function as electron donors to the photooxidized Chl+ formed by the initial photon-driven electron transfer. These tyrosines are both involved in PCET reactions. Methods of molecular biology, synthetic chemistry, and hybrids of the two can provide newly engineered motifs for exploring PCET in tyrosyl radical systems. We will study bacterial reactions centers engineered to generate tyrosine radicals, synthetic tyrosine model compounds with pendant bases to facilitate intramolecular proton transfer, and protein complexes (PSII and ribonucleotide reductase) incorporating chemically modified tyrosines to probe details of tyrosine-based PCET reactions.